This invention relates to a method for welding metal tubes and components. More particularly, the present invention relates to a method of affecting the weld bead geometry by continuously decreasing the average pulsed electric current supplied to an orbital welding device during the welding process, to compensate for the temperature increase experienced in the metal components.
When butt welding together two metal components with tubular extensions, using an orbital welding device, such as the device disclosed in U.S. Pat. No. 5,196,664 to McGushion (1993), it is generally a desired result to create a weld bead which completely penetrates the walls of the tubular extensions, has a weld bead which fully covers the inner diameter and outer diameter seam formed between the two tubular components, and joins together the complete wall cross sectional areas of the same tubular extensions, while maintaining a consistent weld bead geometry.
During the welding process using a constant average current, the average temperature of the components being welded tends to rise due to the thermal energy introduced by the welding device. As the orbital welding device orbits the seam between the components being welded, melting the parent material, the thermal energy is stored in the components as internal energy; while additional thermal energy is added as the remainder of the seam is welded, again still increasing the average component temperature. If the current is not decreased, a surplus of internal energy will accumulate, increasing weld bead and adjacent area temperature to an unacceptable level, causing additional parent material to melt. The weld bead, as a result, will increases to an unacceptable width in some areas, possibly collapsing or rupturing, while possibly being under penetrated in other areas. Therefore, if a constant current is applied to an orbital weld, it is likely that the components will overheat, causing the weld bead to be enlarged to an unacceptable point.
One method currently being used to address the problem of overheating induced irregular weld beads, involves reducing the electric current being provided to the welding device, in a step function manner, throughout the welding process. Usually, when using this single orbit current step reduction method, the orbital welding device's electrode completes at least one welding orbit around the tubular components, but less than two complete welding orbits. The total time required to complete a weld is divided into any number of sectors. Although the time can be divided into numerous sectors, in the example presented the total time will be divided into five sectors.
The first full orbit around the tube is divided into four, generally equal, sectors; and the fifth sector of the weld overlaps and travels beyond the weld start point while the current provided to the electrode is gradually reduced to zero or close to zero. The five sectors generally complete more than one and less than two complete welding orbits.
In the first sector (beginning at the weld start point), the electric arc is first applied to the tubes being welded. After the arc is applied, the electrode will generally remain stationary at this starting point until the walls of the tubes, at that point, are molten from the outer wall surface to the inner wall surface. Then, the electrode will start its rotation through sector one, fusing the tubes in its wake. The average temperature of the tubes will begin to increase, generally in a near linear manner, as soon as the welding process begins. To prevent overheating in section two, due to the heat applied to section one, the current is reduced in a step-like manner when entering sector two.
Even though the current is being reduced, the average temperature of the tubes still increases. To prevent overheating in sector three, due to the heat applied to sectors one and two, the current is again reduced in a step-like manner when entering sector three.
Likewise, to prevent overheating in sector four, due to the heat applied to sectors one, two, and three, the current is once again reduced in a step-like manner when entering sector four. Finally, when the electrode has made one complete weld rotation, past the start point and into sector five, the current level is gradually tapered down to approximately zero in the down slope stage, creating a weld bead width that also tapers to a point.
One of the primary disadvantages of the single orbit current step reduction method is that it is, quite often, a difficult and time consuming process to program an effective weld schedule for the electronic controlling means of the orbital welding device. The welding practitioner must produce numerous, possibly hundreds, of test welds to find the weld schedule which produces the most desirable weld bead. Each test weld bead is inspected for areas of overheating (where the weld bead is too wide) and areas of under-heating (where the weld bead is too narrow or does not fully penetrate the walls of the tubes). After the inspection, the welding practitioner must hypothesize, generally from experience, on which sections it is necessary to increase the current, which sections it is necessary to decrease the current, or if anything at all should be adjusted.
Because, from mere observation, it is not always predictable how exactly the heat generation in any one section relates to a temperature rise in the other sections, it is difficult for the practitioner to calculate which sections need current adjustment in order to effect the geometry of the weld bead in another section. For example, a current adjustment in one section might be programmed to correct a problematic weld bead geometry in another section observed after a test weld; and the following test weld could reveal that the weld bead in the previously problematic section has been corrected, but the weld bead geometry in another previously unaffected section has been negatively affected by the adjustment. A welding practitioner may spend many hours repeatedly correcting the weld bead geometry in one section, only to find that the weld bead geometry in another section has been adversely affected. This long cycle of repetitive correcting becomes even more complicated if the weld schedule is divided in to yet more sections.
Another disadvantage of the current step reduction method is an inconsistent weld bead. At each point that an instantaneous reduction in electric current is programmed, there can be seen on the weld bead of the tubes a corresponding point of immediate weld bead width reduction. As the observer rotates the tubular component to view the entire weld area, the weld bead width appears to start from a minimum width; and steadily increases in width until the point at which the current is stepped down. At the instantaneous current reduction point, the weld bead width is immediately reduced, forming a corner, potentially a stress riser. This weld bead width growth and reduction pattern repeats for every step down reduction in the welding current. This undesirable effect can be reduced by increasing the number of current reduction steps in a given weld schedule. However, increasing the number of current reduction steps significantly complicates the task of programming a suitable weld schedule. Another even longer cycle of repetitive correcting would be necessary.
Yet, another method that is used to attempt a solution to the problem of overheating induced irregular weld beads is the multiple orbit constant current method. Instead of pausing the electrode when the weld first begins, to allow the weld bead to fully penetrate the walls of the tubes (as in the single orbit current step reduction method), the electrode immediately begins to orbit the tubes as soon as an electric arc between the electrode and tubes is established.
As the average temperature of the tubes and their heat energy both increase during the welding process, the molten weld bead penetrates progressively, in a spiral-like fashion, deeper into the walls of the tubes. The first orbit alone will generally not produce a weld bead that has fully penetrated the complete inner seam between the tubes. Therefore, more than one, but usually more than two, orbits by the electrode is required. In one sample weld, as the electrode begins its second welding orbit, the stored heat energy in the tubes allows the molten weld bead to fully penetrate the walls of the tubes in approximately the first quadrant of the second orbit.
The electrode then continues in its second orbit into further sections of the orbit that are more fully penetrated (a result of the first orbit) than the first section. Because the current is constant, there is no way to prevent overheating in these areas of more complete penetration. It is possible that the resulting weld bead characteristics in the first approximate quadrant are acceptable, but the overheated areas would be characterized by an undesirably large, over-penetrated weld bead. Because it is a desired result to produce a weld bead with constant penetration and geometrical characteristics, the multiple orbit constant current method is not usually the best option.
Accordingly, there is a need for a method to easily produce a weld bead with desirable constant penetration and geometrical characteristics, without the need for an overly complicated weld schedule and unnecessarily lengthy test welds.